A rotary positive-displacement machine is a machine that displaces a fluid by means of rotary motion. Rotary positive-displacement machines may include rotary positive-displacement pumps and rotary positive-displacement compressors.
Compressors in general may be used in a wide variety of industries (for example, oil and gas, transportation and refrigeration) to compress a variety of compressible fluids.
One known type of compressor is a screw compressor, in which two members each having a screw thread relatively rotate such that the screw threads intermesh.
It is known to design screw compressors in which each of the members has a conical geometry. Such a screw compressor may comprise a substantially conical inner element having helical grooves and lands on its outer surface, and an outer element having a substantially conical cavity having corresponding helical grooves and lands on its inner surface, such that the grooves and lands intermesh on rotation. The intermeshing grooves and lands may form continuous lines of sealing between the inner element and the outer element, forming a number of closed chambers. The grooves and lands may also be referred to as teeth, gears, threads or lobes.
In operation, a compressible fluid enters the assembly at the large end of the cone. As the inner member and outer member rotate, each of the closed chambers reduces in size as it travels from the large end to the small end of the cone, thereby compressing the compressible fluid. High-pressure fluid leaves the assembly at the small end of the cone.
One example of a screw compressor is detailed in U.S. Pat. No. 2,085,115. The compressor or pump in U.S. Pat. No. 2,085,115 comprises at least three helical gear elements positioned inside one another. The three helical elements may be considered as an outer, a middle, and an inner element. One may consider two groups of mating elements: a first group comprising the outer element and the middle element, and a second group comprising the middle element and the inner element.
In each group of two mating elements, the element with the outer screw surface has one tooth less than the second element surrounding the first element. That is, the middle element has one tooth less than the outer element, and the inner element has one tooth less than the middle element.
It may be important for achieving high efficiency of compressor operation that there is a tight contact between the compressor elements. Complexity of motion of the elements of a compressor, simultaneous interaction of multiple elements which are inserted into each other, and interaction of geometrically complex surfaces may present difficulties in achieving a tight contact between the compressor elements.
Compressor elements may be in contact with each other, and exert force on each other, along complex geometric lines of contact that may extend over the entire surface of the elements along the longitudinal axis (lines of contact that may wrap around the surface of the cone and may extend from one end of the cone to the other). In such cases, it is possible that errors may occur due to imperfections in manufacturing and/or due to backlash. Errors due to manufacturing and/or backlash may lead to imperfect movement of the compressor elements and to imperfect geometry of the lines of contact. In such circumstances, it is possible that the complexity of movement, imperfections in the movement, and forces distributed along imperfect lines of contact may cause the elements to become stuck and cease to rotate. Moreover, at high pressure it may be difficult to keep tight contact between the elements without increasing friction and wear on the elements.
It may be a complex matter to manufacture the surfaces of compressor elements with sufficient precision to ensure tight simultaneous contact between multiple elements of the compressor, where each element of the compressor has a complex geometric surface in the form of a conical spiral.
If one element is driven by the other element, much or all of the torque load may fall on the compressor screw elements, where one screw element is supposed to rotate another screw element. The torque load on the compressor screw elements may lead to an increased frictional force, and therefore to high wear of the compressor screw elements.
A further example of a conical screw compressor is, known from U.S. Pat. No. 1,892,217. A compressor or pump in accordance with U.S. Pat. No. 1,892,217 comprises two helical elements, an inner element inserted into an outer element, where the outer element has one more helical tooth than the inner element. Each tooth of the inner element has a form such that the tooth may maintain constant contact with the outer element at any cross-section. The screw compressor of U.S. Pat. No. 1,892,217 may be made in a cylindrical form or in a conical form.
In some compressor designs, the inner element makes an eccentric rolling motion within a static outer element. The centre of mass of the inner element therefore fluctuates around the central axis of the outer element. The fluctuation of the centre of mass of the inner element around the central axis of the outer element may cause vibration and noise.
In circumstances in which the inner element revolves using an eccentric rolling motion, the axis of the inner element has variable position. The distance from the centre of the inner element to the shaft of the motor is constantly varying. The varying distance from the centre of the inner element to the shaft of the motor may require that an additional device is used between the axis of the motor and the axis of the inner element to smoothly transfer torque from the motor to the inner element.
Because of the fluctuations of the axis of the inner element, the inner element may hit the outer element which may naturally reduce the service period of the compressor.
Another design of a screw compressor is known from PCT Patent Application WO 2008/000505. WO 2008/000505 describes a Moineau pump which has an outer element and an inner element, where the inner element is located inside the outer element. The outer and inner element each have a conical shape, and the elements can revolve around their longitudinal axes. Revolution of the inner element drives the rotation of the outer element or vice versa.
In some compressor designs in which revolution of one element drives the rotation of the other element, much or all of the torque load may fall on the lines of contact between the elements. In some circumstances, the application of such a torque load to the lines of contact between the element may result in high wear of the contacting surfaces, backlash, and excess clearance between the elements. Since compression of gaseous fluids may demand tight contact between the mating surfaces of the compressor's elements, increased clearances (for example, increased clearances caused by wear) may lead to a degraded efficiency of compression.
WO 2008/000505 describes compressor designs in which the inner element or outer element is designed to move along its longitudinal axis. Such movement along a longitudinal axis changes the relative longitudinal positioning of the inner element and outer element.
However, if at least one of the inner element and outer element moves along its axis, gaps between the helical teeth and grooves of the inner element and outer element can occur and gaseous fluid may leak through these gaps.